Rotated STI diode on FinFET technology

ABSTRACT

A diode includes a first plurality of combo fins having lengthwise directions parallel to a first direction, wherein the first plurality of combo fins comprises portions of a first conductivity type. The diodes further includes a second plurality of combo fins having lengthwise directions parallel to the first direction, wherein the second plurality of combo fins includes portions of a second conductivity type opposite the first conductivity type. An isolation region is located between the first plurality of combo fins and the second plurality of combo fins. The first and the second plurality of combo fins form a cathode and an anode of the diode. The diode is configured to have a current flowing in a second direction perpendicular to the first direction, with the current flowing between the anode and the cathode.

BACKGROUND

It is well known that extremely high voltages can develop in thevicinity of an integrated circuit due to the build-up of static charges.A high electrical potential may be generated to an input or outputbuffer of an integrated circuit, which may be caused by a persontouching a package pin that is electrically connected to the input oroutput buffer. When the electrostatic charges are discharged, a highcurrent is produced at the input and output nodes of the integratedcircuit. Electro-Static Discharge (ESD) is a serious problem forsemiconductor devices since it has the potential of destroying theentire integrated circuit.

The duration of the ESD transient is very short, typically in the orderof nanoseconds, and the conventional circuit breakers cannot reactquickly enough to provide adequate protection. For this reason, it hasbecome a known practice to incorporate ESD devices in integratedcircuits. When an ESD transient occurs, the ESD devices are turned on toconduct the ESD current to electrical ground, and hence the integratedcircuits connected to the ESD devices are protected.

There is a plurality of types of ESD devices. Among the commonly usedESD devices are Shallow Trench Isolation (STI) diodes. STI diodes areapplicable solutions for over-drive applications, in which the maximumendurable voltages of gate dielectrics of FinFET may be lower than thepower supply voltages. The chip area consumed by STI diodes, however, isrelatively high.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIG. 1 illustrates a top view of a Shallow Trench Isolation (STI) diodein accordance with some exemplary embodiments, wherein dummy gates areused to define epitaxy regions;

FIGS. 2 through 4 are top views of intermediate stages in themanufacturing of the STI diode in FIG. 1 in accordance with someexemplary embodiments;

FIGS. 5 and 6 are cross-sectional views of the STI diode in FIG. 1;

FIG. 7 illustrates an STI diode, whose fins are merged;

FIG. 8 illustrates a top view of an STI diode in accordance withalternative embodiments, wherein epitaxy block regions are used todefine epitaxy regions;

FIGS. 9 through 12 are top views of intermediate stages in themanufacturing of the STI diode in FIG. 8 in accordance with someexemplary embodiments;

FIGS. 13 and 14 are top views of the STI diodes in accordance withalternative embodiments, wherein dummy gates and epitaxy block regionsare mixed to define epitaxy regions; and

FIGS. 15 and 16 illustrate cross-sectional views of the structure shownin FIG. 8 in accordance with some embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable concepts that can be embodied in a wide varietyof specific contexts. The specific embodiments discussed areillustrative, and do not limit the scope of the disclosure.

A Shallow Trench Isolation (STI) diode and the method of forming thesame are provided in accordance with various exemplary embodiments. Theintermediate stages of forming the STI diode are illustrated. Thevariations and the operation of the STI diodes are discussed. Throughoutthe various views and illustrative embodiments, like reference numbersare used to designate like elements.

FIG. 1 illustrates a top view of STI diode 100 in accordance with someexemplary embodiments. STI diode 100 may be used for Electro-StaticDischarge (ESD) protection, and hence the current Ion when diode 100 isoperated is high. Diode 100 includes combo semiconductor fins 21 and 31separated from each other by STI regions 40. FIG. 5 illustrates across-sectional view of diode 100, wherein the cross-sectional view isobtained from the plane crossing line 5-5 in FIG. 1. As shown in FIGS. 1and 5, combo semiconductor fins 21 (FIG. 1) include a plurality ofn-type semiconductor fins 22 (FIG. 5), which may have an n-type impurityconcentration higher than about 10¹⁹/cm³, or between about 10¹⁹/cm³ andabout 10²¹/cm³. Accordingly, semiconductor fins 22 are also referred toas n+ fins 22. Referring back to FIG. 1, combo semiconductor fins 21further include fins 48A connected to, and in contact with, fins 22.Combo semiconductor fins 31 includes a plurality of p-type semiconductorfins 32 (also refer to FIG. 5), which may have a p-type impurityconcentration higher than about 10¹⁹/cm³, or between about 10¹⁹/cm³ andabout 10²¹/cm³. Accordingly, semiconductor fins 32 are also referred toas p+ fins 32. As shown in FIG. 1, combo semiconductor fins 31 furtherinclude fins 48B connected to, and in contact with, fins 32.

Referring to FIG. 5, p-well region 42 is underlying and connected tosemiconductor fins 22 and 32. Accordingly, p-well region 42 forms a p-njunction (hence diode 100) with n+fins 22, and p+fins 32 may act as thepick-up regions of p-well region 42. N+fins 22 act as the cathode of STIdiode 100, and p+fins 32 and p-well region 42 in combination form theanode of STI diode 100. Furthermore, p-well region 42 comprises aplurality of semiconductor strips 44, each aligned to, and connected to,one of n+fins 22 and p+fins 32. Each of semiconductor strips 44 isbetween two of STI regions 40. N+fins 22 and p+fins 32 (and combo fins21 and 31 in FIG. 1) are spaced apart from each other by a large STIregion 40, which is also denoted as 40A. In some embodiments, length L′of STI region 40A is between about 134 nm and about 806 nm. It isappreciated that although the illustrated exemplary STI diode 100 adoptsp-well region 42, p-well region 42 may be replaced by an n-well regionin alternative embodiments.

Referring back to FIG. 1, a plurality of dummy gates 46 (including 46Aand 46B) are formed over portions of semiconductor fins 48, wherein n+fins 22 and p+ fins 32 are formed between, and defined by, neighboringdummy gates 46. In accordance with some embodiments, different fromfunctional gates, dummy gates 46 are not used to control the currentflowing between the fin portions on opposite ends of dummy gates 46.Dummy gates 46 may be electrically floating, or may be grounded or atany bias. N+ fins 22 and p+ fins 32 may be formed using epitaxy, anddummy gates 46 help relieve the pattern loading effect in the epitaxy ofn+ fins 22 and p+ fins 32. Contact plugs 50 are formed to electricallyconnect to n+ fins 22. Contact plug 52 is formed to interconnect contactplugs 50. Accordingly, all n+ fins 22 are electrically interconnected toform the cathode of STI diode 100. Contact plugs 54 are formed toelectrically connect to p+ fins 32. Contact plug 56 is formed tointerconnect contact plugs 54. Accordingly, all p+ fins 32 areelectrically interconnected to form the cathode of STI diode 100.

As shown in FIG. 1, the lengthwise direction of combo fins 21 and 31 areparallel to the X direction, the likely conducting current I of STIdiode 100 is in the Y direction, which is perpendicular to the Xdirection. In some embodiments, when STI diode 100 is turned on toconduct current I, for example, when an ESD event occurs, and STI diode100 is operated under the forward mode, current I flows from p+ fins 32through the underlying semiconductor strips 44A (FIG. 5), the portion ofp-well 42 under STI region 40A, semiconductor strips 44B, and to n+ fins22. Accordingly, STI Diode 100 is referred to as a rotated STI diodesince the lengthwise direction of combo fins 21 and 31 is perpendicularto the direction of current I. As shown in FIG. 1, the width of currentI is essentially equal to length L of combo fins 21 and 31. Therefore,the width of current I has a great value, and the current conductingability of STI diode 100 is higher than the conducting ability ofconventional STI diodes, whose lengthwise direction of active regionsare parallel to the respective current flowing directions.

FIGS. 2 through 4 illustrate a brief process for forming the structurein FIG. 1. Referring to FIG. 2, semiconductor fins 48, which are higherthan the top surface of STI regions 40, are formed first. Semiconductorfins 48 include fins 48A parallel to each other, and fins 48B parallelto each other, and parallel to fins 48A. The cross-sectional view offins 48 may be found in FIG. 6, wherein the cross-sectional view isobtained from the plane crossing lines 6-6 in FIGS. 1 and 2. As shown inFIG. 6, p-well region 42 may extend into semiconductor strips 44A and44B, semiconductor fins 48A and 48B, and under STI region 40A. Hence,semiconductor fins 48A and 48B may be moderately doped with the p-typeimpurities, for example, with a p-type impurity concentration betweenabout 10¹⁴/cm³ and about 10¹⁸/cm³. Fins 48A are aligned to theunderlying semiconductor strips 44A. Fins 48B are aligned to theunderlying semiconductor strips 44B.

Referring back to FIG. 2, semiconductor fins 48 have lengthwisedirections parallel to the X direction. A plurality of dummy gates 46are formed on the sidewalls and the top surfaces of semiconductor fins48. Dummy gates 46 (including 46A and 46B) may have lengthwisedirections in the Y direction. Dummy gates 46A may be disconnected fromdummy gates 46B. Furthermore, fins 48A may be aligned to fins 48B in theY direction, and dummy gates 46A may be aligned to dummy gates 46B, witheach of dummy gates 46A aligned to one of dummy gates 46B, and with bothaligned to a straight line. Some of dummy gates 46A may be aligned to,and cover, the edges of fins 48A, and some of dummy gate electrode(s)46A may cross, and cover, the intermediate portions (such as the middleportions) of fins 48A. Some of dummy gates 46B may be aligned to theedges of fins 48B, and some of dummy gate electrode(s) 46B may crossover the intermediate portions (such as the middle portions) of fins48B.

A cross-sectional view of dummy gates 46 is shown in FIG. 6, wherein thecross-sectional view is obtained from the plane crossing lines 6-6 inFIGS. 1 and 2. Dummy gates 46 may include dummy gate electrodes 58, anddummy gate dielectrics 60 underlying dummy gate electrodes 58. Dummygate dielectrics 60 are formed on the sidewalls and the top surfaces ofsemiconductor fins 48.

Next, a mask is formed to cover fins 48B and dummy gates 46B, while fins48A are left not covered. FIG. 3 illustrates the respective mask 62.Mask 62 may be a hard mask, and may comprise silicon nitride, forexample. The portions of fins 48A shown in FIG. 2, which portions arenot covered by dummy gates 46A, may be etched, and an epitaxy may beperformed to regrow semiconductor fins 22 where fins 48A were etched.Fins 22 may comprise silicon, silicon carbon, or the like, and may beheavily doped with an n-type impurity (such as phosphorus and/orarsenic) during the epitaxy. In alternative embodiments, fins 22 may beimplanted with the n-type impurity after the epitaxy. The portions offins 48A under dummy gates 46A are not etched, and may be used as theseeds for growing semiconductor fins 22. Fins 22 and the remainingportions of fins 48A form combo fins 21. After the re-growth of fins 22,mask 62 is removed.

Next, a mask is formed to cover fins 22 and dummy gates 46A, while fins48B and dummy gates 46B are left not covered. FIG. 4 illustrates therespective mask 64. Mask 64 may be a hard mask, and may comprise siliconnitride, for example. The portions of fins 48B in FIG. 3, which portionsare not covered by dummy gates 46B, may be etched, and an epitaxy isperformed to regrow semiconductor fins 32 where fins 48B were etched.Fins 32 may comprise silicon germanium, and may be heavily doped with ap-type impurity (such as boron and/or indium) during the epitaxy. Inalternative embodiments, fins 32 are implanted after the epitaxy. Theportions of fins 48B under dummy gates 46B are not etched, and may beused as the seeds for growing semiconductor fins 32. Fins 32 and theremaining portions of fins 48B form combo fins 32. After the re-growthof fins 32, mask 64 is removed.

Next, as shown in FIG. 1, contact plugs 50, 52, 54 and 56 are formed toconnect to fins 22 and 32. Silicide regions (not shown) may be formed onthe top surfaces of fins 22 and 32. Contact plug 52 and 56 are used asthe connections of the cathode and the anode, respectively, of STI diode100.

In STI diode 100 shown in FIG. 1, fins 22 and 32 are separate andparallel fins. In alternative embodiments, as shown in FIG. 7, fins 22may be merged after epitaxy, and fins 32 may be merged after epitaxy.Semiconductor strips 44A and 44B are shown using dashed lines toindicate the original positions of the etched portions of fins 48A and48B, respectively. The formation of STI diode 100 in FIG. 7 isessentially the same as the formation of diode 100 in FIG. 1, as shownin FIGS. 2 through 4, except the volumes of fins 22 and 32 in FIG. 7 aregreater than in FIG. 1.

FIGS. 8 through 14 illustrate the final structures and the formation ofvarious STI diodes 100 in accordance with alternative embodiments.Unless specified otherwise, the materials and formation methods of thecomponents in these embodiments are essentially the same as the likecomponents, which are denoted by like reference numerals in theembodiments shown in FIGS. 1 through 7. The details regarding theformation process and the materials of the components shown in FIGS. 8through 14 may thus be found in the discussion of the embodiments shownin FIGS. 1 through 7.

Referring to FIG. 8, in accordance with some embodiments, STI diode 100includes fins 22 connected to fins 48A to form combo fins 21, and fins32 connected to fins 48B to form combo fins 31. Combo fins 21 and 31 arealigned to the underlying semiconductor strips 44 (also see FIGS. 5 and6 for reference). No dummy gate electrodes are formed over fins 48,however, and dielectric layers (not shown) such as a Contact Etch StopLayer (CESL) and an Inter-Layer Dielectric (ILD) will be formed on thesidewalls and top surfaces of fins 48, with the CESL contacting fins 48in some embodiments. Contact plugs 50 and 52 are formed in the ILD tointerconnect fins 22, and contact plugs 54 and 56 are formed in the ILDto interconnect fins 32. STI diode 100 in these embodiments is also arotated STI diode since the direction of current I of diode 100 isperpendicular to the lengthwise directions of combo fins 21 and 31.

In FIG. 8, epitaxy block regions 66 are illustrated. The relativepositions of epitaxy block regions 66 (with relative to combo fins 21and 31) may be essentially the same as the locations of dummy gateelectrodes 46 in FIG. 1. Epitaxy block regions 66 are the regions thatare blocked during both the epitaxial re-growth of fins 22 and theepitaxial re-growth of fins 32, hence are the regions in which theoriginal fins 48 remain. FIGS. 9 through 12 schematically illustrate aprocess for forming STI diode 100 in FIG. 8.

Referring to FIG. 9, which is a top view, fins 48A are formed to alignto underlying semiconductor strips 44A. Fins 48B are formed to align tounderlying semiconductor strips 44B. Fins 48A and 48B are parallel toeach other. Fins 48A and 48B are above the top surfaces of STI regions40. Fins 48A and 48B may have a same doping concentration as theunderlying well region (not shown), which may be essentially the same asp-well 42 in FIG. 6. In some exemplary embodiments, fins 48A and 48B andthe underlying well region are moderately doped, for example, to ap-type impurity concentration between about 10¹⁴/cm³ and about 10¹⁸/cm³.Throughout the description, the regions moderately doped with a p-typeimpurity are referred to as moderately doped p-type regions. Hence, fins48A and 48B are moderately doped p-type fins.

Referring to FIG. 10, mask 62 is formed. Mask 62 may be a hard maskcomprising silicon nitride, for example, although other materials thatcan sustain the high temperature of epitaxy may also be used. Mask 62covers fins 48B and some portions of fins 48A, wherein the maskedportions of fins 48A correspond to epitaxy block regions 66. The exposedportions of fins 48A are then etched, and fins 22 are re-grown in anepitaxy step. Fins 22 may be in-situ heavily doped with an n-typeimpurity (such as phosphorus and/or arsenic) during the epitaxy, or maybe implanted after the epitaxy.

Next, mask 62 is removed, and mask 64 is formed, as shown in FIG. 11.Mask 64 may also be a hard mask comprising silicon nitride, for example,although other materials may also be used. Mask 64 covers fins 48A and22 and some portions of fins 48B, wherein the masked portions of fins48B correspond to epitaxy block regions 66. The exposed portions of fins48B are then etched, and fins 32 are re-grown in an epitaxy step. Fins32 may be heavily doped with a p-type impurity (such as boron and/orindium) during the epitaxy, or may be implanted after the epitaxy. Afterthe re-growth of fins 32, mask 64 is removed. The resulting structure isshown in FIG. 12. A CESL (not shown), an ILD (not shown), and contactplugs 50, 52, 54, and 56 may then be formed, and the resulting STI diode100 is shown in FIG. 8.

FIGS. 15 and 16 illustrate cross-sectional views of the structure shownin FIG. 8. The cross-sectional view shown in FIG. 15 is obtained fromthe plane containing line 15-15 in FIG. 8. The cross-sectional viewshown in FIG. 16 is obtained from the plane containing line 16-16 inFIG. 8.

FIG. 13 illustrates STI diode 100 in accordance with alternativeembodiments. In these embodiments, dummy gate electrodes 46A are formedto cover some portions of fins 48A, and to assist the epitaxy growth offins 22. Accordingly, dummy gate electrodes 46A define the locationswhere fins 22 are to be re-grown. The formation of dummy gate electrodes46A is similar to the formation of dummy gate electrodes 46A shown inFIG. 2. The re-growth of fins 22 is similar to what is shown in FIG. 3.The locations of fins 32, on the other hand, are defined by epitaxyblock regions 66, wherein no dummy gate electrode is formed to coverfins 48B. The formation of fins 32 is similar to what is shown in FIG.11.

FIG. 14 illustrates STI diode 100 in accordance with yet alternativeembodiments. These embodiments are similar to the embodiments in FIG.13, except that dummy gate electrodes 46B are formed to cover someportions of fins 48B, and to assist the epitaxy growth of fins 32, whileepitaxy block regions 66 are used to assist the epitaxy growth of fins22, wherein no dummy gate electrode is formed to cover fins 48A.

Both the STI diodes 100 in FIGS. 13 and 14 include dummy gate electrodes46, which may result in the reduction of pattern loading effect in theformation of gate electrodes in the same wafer. When used in ESDprotection applications, p+ fins 32 may be connected to the electricalground, and n+ fins may be connected to high voltages (while chip isunder normal operation), which are also known as over-drive voltageshigher than what the dummy gate dielectrics 60 (FIG. 6) are designed tosustain. Accordingly, the STI diode 100 in FIG. 14 may be used to workwith applications having over-drive voltages. On the other hand, when noover-drive voltages are used, the STI diodes 100 shown in both FIGS. 13and 14 may be used.

In each of the embodiments in FIGS. 1, 7, 8, 13, and 14, each of thecombo fins forming the cathode and the anode of STI diode 100 includes aplurality of portions having different impurity concentrations and/ordifferent conductivity types. For example, the combo fins 21 forming thecathode may include fins 22, which are n+ regions, and fins 48A, whichare moderately doped p-type regions. Fins 22 and fins 48A are aligned toand in contact with each other to form combo fins 21. The combo fins 31forming the anode may include fins 32, which are p+ regions, and fins48B, which are moderately doped p-type regions. Fins 32 and fins 48B arealigned to and in contact with each other to form combo fins 31.

In the embodiments of the present disclosure, fin structures are adoptedto form STI diodes, which may be used for ESD protection. Theanode-to-cathode (or cathode-to-anode) current flowing through STIdiodes 100 (FIGS. 1, 7, 8, 13, and 14) are perpendicular to thelengthwise directions of the fins, and hence the current-conductingability of the STI diodes may be increased due to the increased widthsof the current paths. Furthermore, since the dummy gate electrodes andthe epitaxy block regions are perpendicular to the fins, the lengths ofthe dummy gate electrodes and the epitaxy block regions are likely to bereduced, and the chip area occupied by the dummy gate electrodes and theepitaxy block regions may be reduced, and hence the resulting STI diodesmay occupy less chip area.

In accordance with some embodiments, a diode includes a first pluralityof combo fins having lengthwise directions parallel to a firstdirection, wherein the first plurality of combo fins comprises portionsof a first conductivity type. The diodes further includes a secondplurality of combo fins having lengthwise directions parallel to thefirst direction, wherein the second plurality of combo fins comprisesportions of a second conductivity type opposite the first conductivitytype. An isolation region is located between the first plurality ofcombo fins and the second plurality of combo fins. A well region isunderlying the isolation region, the first plurality of combo fins, andthe second plurality of combo fins. The well region and the firstplurality of combo fins form a first one of a cathode and an anode ofthe diode. The second plurality of combo fins forms a second one of thecathode and the anode. The diode is configured to have a current flowingin a second direction perpendicular to the first direction, with thecurrent flowing between the anode and the cathode.

In accordance with other embodiments, a diode includes a p-well region,an STI region, a first semiconductor strip, and a first and a secondsemiconductor strip contacting opposite sidewalls of the STI region andparallel to each other. The first and the second semiconductor stripsare portions of the p-well region. A first combo fin overlaps the firstsemiconductor strip, wherein the first combo fin includes a firstportion heavily doped to p-type, and a second portion in physicalcontact with the first portion, wherein the second portion is moderatelydoped to p-type. A second combo fin overlaps the second semiconductorstrip. The second combo fin includes a third portion heavily doped ton-type, and a fourth portion in physical contact with the third portion.The fourth portion is moderately doped to p-type. The first combo finand the p-well region form an anode of the diode. The second combo finforms a portion of a cathode of the diode.

In accordance with yet other embodiments, a method includes forming afirst and a second semiconductor fin, wherein the first and the secondsemiconductor fins overlap a first and a second semiconductor strip,respectively. The first and the second semiconductor strips are parallelto each other, and contact opposite edges of an STI region. A wellregion of a first conductivity type is formed to extend under the STIregion, into the first and the second semiconductor strips, and into thefirst and the second semiconductor fins. The second semiconductor finand a first portion of the first semiconductor fin are covered. A secondportion of the first semiconductor fin is etched. A semiconductormaterial is re-grown in a space left by the second portion of the firstsemiconductor fin to form a third portion for the first semiconductorfin. The third portion is heavily doped to the first conductivity type.The first semiconductor fin and a fourth portion of the secondsemiconductor fin are covered. A fifth portion of the secondsemiconductor fin is etched. An additional semiconductor material isre-grown in a space left by the fifth portion of the secondsemiconductor fin to form a sixth portion for the second semiconductorfin. The sixth portion is heavily doped to a second conductivity typeopposite the first conductivity type. The first and the secondsemiconductor fins form a diode.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A diode comprising: a first plurality of combofins having lengthwise directions parallel to a first direction, whereinthe first plurality of combo fins comprises portions heavily doped top-type, and each of the first plurality of combo fins comprises: a firstportion heavily doped with an impurity of the p-type; and a secondportion in physical contact with the first portion, wherein the secondportion is moderately doped with an impurity of the p-type; a firstdummy gate electrode covering the second portion of a first fin amongthe first plurality of combo fins; a second plurality of combo finshaving lengthwise directions parallel to the first direction, whereinthe second plurality of combo fins comprises portions heavily doped ton-type; an isolation region between the first plurality of combo finsand the second plurality of combo fins, and wherein both the firstportion and the second portion comprise a portion higher a top surfaceof the isolation region; and a well region of p-type, wherein the wellregion and the first plurality of combo fins form a first one of acathode or an anode of the diode, and the second plurality of combo finsforms a second one of the cathode or the anode, and wherein the diode isconfigured to have a current flowing in a second direction perpendicularto the first direction, with the current flowing between the anode andthe cathode, and wherein the diode further comprises a p-n junction overa bottom surface of the isolation region, with the p-n junction beingwhere the anode and the cathode of the diode contact each other, whereinthe well region comprises: a first portion underlying the isolationregion, the first plurality of combo fins, and the second plurality ofcombo fins; and a second portion extending to a level between a topsurface and a bottom surface of the isolation region.
 2. The diode ofclaim 1, wherein the second plurality of combo fins comprises a secondfin, and wherein the diode further comprises: a second dummy gateelectrode covering an end portion of the second fin, wherein the firstdummy gate electrode and the second dummy gate electrode aresubstantially aligned to a straight line.
 3. The diode of claim 1,wherein the second plurality of combo fins comprises a second fin, andwherein the diode further comprises: a first dummy gate electrodecovering an end portion of the first fin, wherein the second fin doesnot have a dummy gate electrode formed thereon.
 4. The diode of claim 2further comprising: a third dummy gate electrode covering an additionalend portion of the first fin, wherein a middle portion of the first finis between the first dummy gate electrode and the third dummy gateelectrode; and a contact plug over and electrically connect to themiddle portion of the first fin.
 5. The diode of claim 2, wherein eachof the second plurality of combo fins comprises: a first portion heavilydoped with an impurity of the n-type; and a second portion in physicalcontact with the first portion, wherein the second portion is moderatelydoped with an impurity of the p-type, wherein the second portion of thesecond fin is covered by the second dummy gate electrode, and whereinboth the first portion and the second portion comprise a portion over atop surface of the isolation region.
 6. The diode of claim 1, whereinthe second portion of the well region comprises a first sidewall and asecond sidewall opposite to each other, with both the first sidewall andthe second sidewall contacting edges of the isolation region.
 7. A diodecomprising: a p-well region; a Shallow Trench Isolation (STI) region; afirst semiconductor strip and a second semiconductor strip contactingopposite sidewalls of the STI region and parallel to each other, whereinthe first and the second semiconductor strips are portions of the p-wellregion; a first combo fin overlapping the first semiconductor strip,wherein the first combo fin comprises: a first portion heavily doped top-type; and a second portion in physical contact with the first portion,wherein the second portion is moderately doped to p-type; and a secondcombo fin overlapping the second semiconductor strip, wherein the firstcombo fin and the p-well region form an anode of the diode, and whereinthe second combo fin forms a portion of a cathode of the diode, andwherein the second combo fin comprises: a third portion heavily doped ton-type; and a fourth portion in physical contact with the third portion,wherein the fourth portion is moderately doped to p-type.
 8. The diodeof claim 7 further comprising: a first dummy gate electrode on sidewallsand a top surface of the second portion of the first combo fin; and asecond dummy gate electrode on sidewalls and a top surface of the fourthportion of the second combo fin.
 9. The diode of claim 7 furthercomprising: a dummy gate electrode on sidewalls and a top surface of thesecond portion of the first combo fin, wherein no dummy gate electrodeis on sidewalls and a top surface of the fourth portion of the secondcombo fin.
 10. The diode of claim 7 further comprising: a dummy gateelectrode on sidewalls and a top surface of the fourth portion of thesecond combo fin, wherein no dummy gate electrode is on sidewalls and atop surface of the second portion of the first combo fin.
 11. The diodeof claim 7, wherein no dummy gate electrode is on sidewalls and topsurfaces of the second portion of the first combo fin and the fourthportion of the second combo fin.
 12. The diode of claim 7, wherein thesecond portion and the fourth portion are end portions of the firstcombo fin and the second combo fin, respectively.
 13. The diode of claim12, wherein a middle portion of the first combo fin is moderately dopedwith a p-type impurity, and wherein opposite ends of the middle portionare connected to heavily doped p-type regions.
 14. The diode of claim 7,wherein the first combo fin further comprises a fifth portion and asixth portion, wherein the fifth portion and the sixth portion aremoderately doped p-type regions, and wherein the fifth portion and thesecond portion cover opposite ends of the first combo fin, and whereinthe sixth portion covers a middle portion of the first combo fin.
 15. Adiode comprising: a first combo fin having lengthwise directionsparallel to a first direction, wherein the first combo fin is of p-type,and acts as an anode of the diode; a first dummy gate electrode coveringa portion of the first combo fin; a second combo fin having lengthwisedirections parallel to the first direction, wherein the second combo finis of n-type, and acts as a cathode of the diode; a second dummy gateelectrode covering a portion of the second combo fin, wherein the firstdummy gate electrode and the second dummy gate electrode are notdirectly electrically connected; an isolation region between the firstcombo fin and the second combo fin, wherein the diode comprises a p-njunction over a bottom surface of the isolation region, wherein thediode is configured to have a current flowing from the anode to thecathode, with the current comprising a portion flowing directlyunderlying the isolation region, and wherein the portion of the currentflows in a second direction perpendicular to the first direction; and ap-well region forming a part of an anode of the diode, wherein thep-well region comprises a portion overlapped by the first combo fin, andthe portion of the p-well region is at a same level as the isolationregion.
 16. The diode of claim 15, wherein the first combo fin comprisesa first portion and a second portion, and the diode further comprises adummy gate dielectric covering the first portion.
 17. The diode of claim16 further comprising a dummy gate electrode overlapping the dummy gatedielectric.
 18. The diode of claim 17, wherein the dummy gate electrodeis electrically grounded.
 19. The diode of claim 15, wherein the portionof the p-well region comprises a first sidewall and a second sidewallopposite to each other, with both the first sidewall and the secondsidewall contacting edges of the isolation region.
 20. The diode ofclaim 15, wherein the first dummy gate electrode and the second dummygate electrode are not directly electrically connected.